Heat exchangers are employed within the automotive industry as radiators which cool the engine coolant, condensers and evaporators for use in air conditioning systems, and heater cores. In order to efficiently maximize the amount of surface area available for transferring heat between the environment and a fluid flowing through the heat exchanger, the design of the heat exchanger is typically of a tube-and-fin type in which numerous tubes thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa. For example, heat exchangers used in the automotive industry as heater cores serve to transfer heat from the engine coolant to the air entering the passenger compartment.
Heat exchangers are increasingly being formed by a brazing operation in which the individual components of the heat exchanger are permanently joined together with a brazing alloy. Generally, brazed heat exchangers are lower in weight and are better able to radiate heat as compared to heat exchangers formed by known mechanical assembly techniques. An example of a brazed heat exchanger is the headered tube-and-center (HTC) type, which utilizes a number of parallel tubes which are brazed to and between a pair of headers, with finned centers being brazed between each adjacent pair of tubes. Conventionally, headered tube-and-center heat exchangers have been constructed by inserting the parallel tubes into apertures formed in each of an opposing pair of headers. A finned center is then positioned between each adjacent pair of parallel tubes. A tank is attached to each header so as to form reservoirs which are in fluidic communication with the tubes through the apertures. One or both tanks include one or more ports which serve as an inlet and outlet to the heat exchanger.
In the automotive industry, copper and brass heater cores which were widely used in the past have largely been replaced by aluminum heater cores in order to reduce the weight of automobiles. To minimize weight, many heater cores are formed to have plastic tanks and aluminum tubes, headers and fins, necessitating that the tanks be bonded to a brazed assembly formed by the headers, tubes and fins. Others are formed entirely from aluminum alloys, enabling the entire heater core to be joined in a single operation. A serious problem with these types of heater cores is that the brazing operation significantly softens the aluminum alloy or alloys which form the heater core. Consequently, an aluminum alloy heater core is subject to shorter service life from erosion of its internal surfaces, especially when solid contaminants are suspended in the coolant, as is often the case.
Erosion particularly occurs due to the coolant directly impinging the ends of the tubes of a headered tube-and-center heater core as it flows into the heater core through the inlet, causing significant erosion of the metal and premature deterioration of the heater core. Furthermore, the performance of the heater core is impaired because the flow distribution of the coolant among the tubes is not uniform, with the tubes closest the inlet handling the majority of coolant flow through the heater core.
The prior art has sought to overcome the abovenoted erosion problem by increasing the wall thickness of the coolant tubes. However, doing so is undesirable in that it increases the weight of the heater core and complicates its manufacture and assembly. For heater cores with plastic tanks, the prior art has also attempted to solve the erosion problem by ultrasonically welding a plastic baffle downstream of the heater core inlet in order to deflect the coolant away from the tube ends. However, this solution undesirably requires an additional assembly and joining step in order to position and ultrasonically weld the baffle to the tank prior to bonding the tank to the header. Furthermore, plastic baffles are not feasible for heater cores formed entirely from aluminum. Manufacturers of all-aluminum heater cores have not sought a solution similar to the plastic baffle of the prior art in that an aluminum baffle would require being welded in place prior to brazing, which would be undesirable and costly from a processing standpoint. In addition, the welding operation might result in warpage or distortion of the components, which would seriously impede the assembly and fixturing of the components for brazing. Finally, high warranty and/or replacement costs would result if the weld were to fail, allowing the baffle to rattle within the heater core.
Consequently, the use of a flow restrictor placed upstream of the inlet to an all-aluminum heater core has been proposed in order to reduce the flow rate through the heater core which, in turn, reduces the tendency for the coolant to erode the ends of the coolant tubes. However, the performance of a heater core can be significantly compromised at reduced flow rates, which in practice have been as little as five gallons per minute or less. Others have suggested mounting the inlet pipe to the side of the tank in order to avoid impinging the inlet flow directly on the coolant tubes. However, this approach typically requires the formation of complex and, therefore, costly bends in the inlet pipe. Furthermore, the space available in a vehicle typically dictates the placement of the inlet and outlet pipes, and will often prevent locating the inlet pipe to the side of the tank. Yet others have suggested increasing the diameter of the inlet pipe in order to reduce the coolant velocity as the coolant enters the heater core. Again, however, the available space in a vehicle may not allow the use of larger diameter pipes. In addition, larger diameter pipes are more costly and necessitate a bend radius which is greater than that possible for a smaller diameter tube. Consequently, the installation of a larger diameter pipe may be complicated or infeasible for a particular application.
From the above, it is apparent that the prior art lacks a suitable solution to the problem of coolant tube erosion in an all-aluminum heat exchanger such as those used as an automotive heater core. Accordingly, it would be desirable to provide an all-aluminum monolithic heat exchanger which is characterized by significantly reduced internal erosion, particularly at the ends of the coolant tubes immediately downstream of the heat exchanger inlet, without reducing the flow capacity of the heat exchanger and without complicating the assembly and installation of the heat exchanger. Furthermore, it would be desirable if such a heat exchanger could be readily formed using a single brazing operation, so as to facilitate its manufacture. It would also be desirable if the efficiency of such a heat exchanger could simultaneously be enhanced by improving the flow distribution of the coolant among the coolant tubes.